The invention relates to the field of broadband amplifiers, and in particular to a variable gain, high frequency, low noise amplifier.
Wireless receivers often receive low power signals at one location and high power at another location. This is particularly true for portable and automotive receivers. To account for the variation in received signal strength, these receivers typically use a low noise amplifier (LNA) to amplify received low power signals while preventing them from being overwhelmed by the internally generated receiver noise. The LNA also receives high power input signals with minimal distortion. Therefore, due to the variation in the power of received signals, the LNA is often a compromise between gain, minimum noise figure (NF) and linearity.
When the receiver is implemented with an integrated circuit (IC) that includes an RF front end on the IC, additional design constraints arise such as DC power consumption, substrate losses and package parasitics that constrain the LNA design options and receiver performance. The ability to adjust the LNA gain is useful for reducing high level input signal distortion in succeeding receiver circuits, such as the mixer without degrading the detection of low level input signals. Adjustable gain permits the use of low DC power consumption, lower linearity mixer and IF amplifier circuitry because high level input signals are not amplified or are even attenuated by the adjustable gain LNA. This increases the IC receiver""s dynamic range, battery lifetime, and ultimately lowers the cost of the receiver.
As the frequency of the received signal increases, the design of a low cost variable gain LNA for an IC receiver becomes more complicated. Materials such as GaAs and other Group III-V compound technologies offer MESFET type transistors which can be used to fabricate ICs with low noise, high linearity properties. GaAs ICs can have variable gain or variable attenuation by using one or more MESFET type devices as voltage controlled variable resistors. Yet in comparison to competing silicon (Si) technologies, such as Si bipolar junction transistor (BJT ) ICs, these technologies often cost more and are less mature in their ability to integrate digital circuitry and other required receiver circuitry such as low phase noise oscillators, dual modulus prescalers and analog-to-digital converters. However, Si BJT devices are not amendable to use as voltage variable resistors. Therefore, to vary gain in a Si BJT device, other circuit techniques such as current steering must be used.
To provide a low cost IC receiver, the IC receiver must include a low cost package such as SOIC and SSOP injection mold plastic packages. However, these packages are generally designed for low frequency analog and digital ICs due. The large lead lengths and bond wire inductance of these packages and low cost assembly techniques generally associated with these packages make them difficult to use in high frequency systems. Specifically, at high frequency these packages generally have greater signal attenuation, poorer RF isolation and grounds. Poor RF isolation and grounding at high frequency make it difficult to achieve a large adjustable gain range from any type of variable gain circuitry, including the current steering method.
Therefore, there is a need for a low cost integrated circuit receiver that includes a variable gain low noise amplifier capable of operating at high frequency.
An object of the present invention is to provide a variable gain high frequency integrated circuit receiver.
A further object is to provide a variable gain high frequency low noise amplifier.
Briefly, according to the present invention, an integrated electronic device includes a variable gain amplifier and a filter. The device receives a high frequency input signal that is input to variable gain amplifier which also receives gain control signals that set the amount of gain applied to the input signal to create an amplifier output signal. The filter comprises passive components including inductors, capacitors and resistors. The filter also includes parasitic inductances provided by package leads and bond leads/wires.
The variable gain amplifier includes a current steering mechanism that is responsive to the gain control signals, and sets the desired amplifier gain in response to the control signals. The gain may be less than one. In a preferred embodiment, the steering mechanism includes parallel connected NPN transistors in cascode that steer the input signal between a load and the filter that isolates the high frequency input signal from the amplifier output signal.
The filter is constructed from package leads, bond wires and dedicated on-chip components that overcome the lack of a precise on-chip RF ground to provide the requisite isolation between the input signal and the amplifier output signal.
Advantageously, the filter of the variable gain high frequency LNA overcomes the lack of a good ground on the IC.
These and other objects, features and advantages of the present invention will become apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.